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Longba, situated in southern Gansu Province, China, occupies a strategic position in the transitional zone between the Tibetan Plateau to the southwest and the Loess Plateau to the northeast, with coordinates at 33.3702°N, 104.9603°E. This city of approximately 2.4 million people lies within the Bailong River valley, a tributary of the Yangtze River system, which carves through rugged mountainous terrain characterized by steep slopes and narrow valleys. The urban area sits at an elevation of around 1,000 meters above sea level, creating a basin-like topography that significantly influences air quality by trapping pollutants under stable atmospheric conditions. Longba's geography places it within the Qinling-Daba Mountains region, surrounded by forested highlands that historically supported agriculture but now face urban expansion pressures. The city serves as a regional hub connecting eastern Sichuan with central Gansu, leading to transportation corridors that contribute to vehicular emissions. Proximity to the Bailong River provides some atmospheric moisture but insufficient to regularly disperse accumulated pollutants. The urban-rural gradient shows dense central development gradually giving way to terraced farmlands on valley slopes, where biomass burning occasionally adds to particulate matter. Industrial activity remains moderate compared to eastern Chinese cities but includes small-scale manufacturing and construction that generate localized pollution. This combination of topographic confinement, growing urbanization, and regional transport networks creates persistent air quality challenges, particularly during periods of meteorological stagnation.

Longba's air quality follows distinct seasonal patterns shaped by its valley geography and East Asian monsoon influences. Spring (March-May) brings increasing temperatures and occasional dust storms from the northwest, mixing with local pollutants to create moderate particulate levels, though rising afternoon winds provide some dispersion; sensitive individuals should monitor days following dust events. Summer (June-August) offers the cleanest air as the East Asian monsoon delivers southeasterly winds and frequent rainfall that effectively scrub pollutants from the atmosphere, making this the optimal season for outdoor activities despite occasional high humidity. Autumn (September-November) sees deteriorating conditions as monsoon retreats and temperature inversions become frequent in the valley, trapping vehicle emissions and construction dust; October typically marks the beginning of the pollution season with reduced ventilation. Winter (December-February) presents the most challenging period as cold, stable air settles in the basin, creating persistent temperature inversions that concentrate heating emissions from coal and biomass, along with limited dispersion from weak valley winds; January often experiences the poorest air quality, requiring vulnerable groups to limit prolonged outdoor exposure. Throughout the year, morning fog can exacerbate pollution by combining with particulate matter, while nighttime cooling in the valley frequently leads to pollution buildup that dissipates only partially during daytime heating cycles.